1. Field of the Invention
The present invention relates to a cell search method and a mobile communication terminal, and more particularly to a cell search method for accessing a network that uses the Wideband Code Division Multiple Access (W-CDMA) system and to a mobile communication terminal that performs the cell search.
2. Description of the Related Art
In a mobile communication system that uses the Code Division Multiple Access (CDMA) system, a radio zone identified with a frequency and Code forms a single communication area (cell) When a plurality of common carriers run their respective Public Land Mobile Networks (PLMNs), each common carrier uses one or two or more frequencies that a reassigned thereto. For example, according to the 3rd Generation Partnership Project (3GPP), the frequency band (FB) of CDMA radio signals from radio base stations to mobile communication terminals is 60 MHz wide, ranging from 2110 MHz to 2170 MHz. Center frequencies can be set at intervals of 200 KHz within a range from 2112.4 MHz to 2167.6 MHz. The number of possible center frequencies is therefore 277, so a search for all available frequencies takes a considerable time. While the available frequency band will be further expanded in the future, shortening the time required to detect frequencies is therefore demanded.
FIGS. 1A and 1B show examples of frequencies used in PLMNs. In the area A shown in FIG. 1A, the PLMN-A of a common carrier A adopts a frequency F01 and the PLMN-B of a common carrier B adopts frequencies F11 and F12. When a mobile communication terminal is connected to the PLMN-A in the area A, the mobile communication terminal stores F01 as a frequency of the PLMN-A. When the mobile communication terminal enters a no service area, e.g., an elevator, the communication with the radio base station is interrupted. However, when the mobile communication terminal stays within the area A, the mobile communication terminal is capable of easily restarting the communication with the radio base station by using the stored frequency F01. When the mobile communication terminal has moved from the area A to an area B, the mobile communication terminal is unable to communicate with a base station of the PLMN-A by using the frequency F01. When the PLMN-A adopts frequencies F02 and F03 in the area B of FIG. 1B, detecting the frequencies F02 and F03 usually requires sequentially searching for all frequencies. Such frequency detecting processing takes a long time and consumes increased power. Also, while the mobile communication terminal searches for all frequencies, the mobile communication terminal may detect, with high received field strengths, frequencies F11 and F13 that are adopted by the PLMN-B. In this case, the mobile communication terminal obtains notifying information sent from a radio base station in order to check the PLMN of the cells of the detected frequencies. In such a case, recovering the communication with the desired PLMN-A takes a still longer time. Such a long-time cell search process considerably affects the time required to become ready for communication after power-on, the time required for the mobile communication terminal to recover from a no service area to a service area, and the time required to recover communication after detection of desynchronization or a state being out of synchronization.
For example, JP2003-348648 A discloses a high-speed cell search method. In this method, the electric power distribution in the whole frequency band is measured and frequency bands to be searched are limited and ranked. A search based on the conditions is conducted to tentatively select a cell. JP 2000-333259 A discloses a technique in which a mobile station limits the number of frequency channels to be scanned in a single scan operation and changes part of the frequency channels for each scan operation.
However, with the known techniques above, when a mobile station detects a frequency having a high received field strength, the mobile station obtains notifying information sent from a radio base station in order to check the PLMN of the cell of that frequency. This considerably increases the time required for the mobile station to recover communication with the desired PLMN.